In radio communications, a signal attenuates greatly in the propagation path between a transmitter and a receiver, and the characteristics of the propagation path fluctuate greatly. Therefore, it has heretofore been performed to dispose an automatic gain control (AGC) amplifier in a reception apparatus and to control a reception signal to an amplitude suitable for a signal processing circuit at the succeeding stage, or to employ an error correction code capable of correcting the error of data, as a transmission code.
Especially, a scheme which employs a convolution code as the error correction code and which performs the maximum-likelihood decoding by Viterbi decoding has a high error correction ability and is utilized in various fields.
Besides, in the Viterbi decoding, the state transition of a decoder is estimated from a reception series, and the most probable (maximum-likelihood) state transition is selected so as to estimate a transmitted information series. On this occasion, the Hamming distance between a received code and a candidate code is generally employed as a likelihood for numerically representing the probable-ness of the state transition. Here, in evaluating the likelihood, there have been known a hard decision which represents the received code by either of binary values “0” and “1”, and a soft decision which represents the received code by any of multiple values corresponding to the amplitude of the reception signal. In recent years, the soft decision of higher error correction ability has been adopted more and more. Hereinbelow, a value which represents the likelihood in the soft decision shall be called “soft decision value”.
Meanwhile, in the reception apparatus of this type, the reception signal is sampled by an A/D converter, and signal processing is executed using the resulting sampling values. It has been known that, in a case where the amplitude of the input signal to the A/D converter is not an appropriate magnitude (for example, in a case where the gain control of the AGC amplifier is not appropriately performed for any reason), a characteristic degradation ascribable to the nonlinearity of the A/D converter occurs (refer to, for example, Manabu Sawada, Hiraku Okada, Takaya Yamazato, Masaaki Katayama: “Influence of the Nonlinearity of the ADC in an OFDM Receiver”, Collection of Preliminary documents of the 10th International Workshop 2005, Hamburg, 2005. 8, p. 220-p. 224).
More specifically, as shown by a conversion characteristic in FIG. 3, the A/D converter has the properties that a continuous analog signal is converted into discrete digital data, and that a signal exceeding an input full scale range is clipped to the maximum value or minimum value of the digital data. In addition, in a case where the amplitude of the input signal of the A/D converter is too small, only part of the input full scale range of the A/D converter is used, and the number of significant bits (resolution) for the input signal is not sufficiently ensured, so that the A/D conversion data contain large quantization errors. To the contrary, in a case where the amplitude of the input signal of the A/D converter is too large, the signal having exceeded the input full scale range of the A/D converter is clipped to incur a waveform distortion. In either case, the characteristic degradation occurs.
Besides, in recent years, the radio communications have been enlarged in capacity, and orthogonal frequency division multiplexing (OFDM) has been known as one of typical transmission schemes for realizing the large-capacity communications.
In the OFDM, information items are respectively carried on a large number of subcarriers of different frequencies, and the resulting subcarriers are multiplexed and transmitted. Therefore, a transmission signal (termed “multicarrier signal”) into which the signals of the individual subcarriers are combined comes to have a very large amplitude (peak power) when the phases of the signals of the individual subcarriers have agreed, and it becomes a signal whose peak to average power ratio (PAPR) is large.
For this reason, in a case where the AGC amplifier is disposed in the reception apparatus adopting the OFDM, the gain of the AGC amplifier (in turn, the amplitude of the input signal to the A/D converter) will not be appropriately controlled at a higher possibility than in the case of a single-carrier signal which is generally employed, with the result that the characteristic degradation ascribable to the nonlinearity of the A/D converter as stated above will occur at a higher possibility.
On the other hand, there has been disclosed a reception apparatus wherein the instantaneous reception amplitude of the reception signal is detected, and the soft decision value for use in the error correction decoding is multiplied by a weighting factor dependent upon the instantaneous reception amplitude of the reception signal, thereby to correct the soft decision value, and wherein the decoding is performed using the corrected soft decision value (refer to, for example, JP-A-H5-315977).
In the conventional apparatus, however, the weighting factors are set so as to obtain the soft decision value whose likelihood of the state transition is low, under the assumption that the reliability of the reception signal will be low (an error will be contained at a high possibility) whenever the instantaneous reception amplitude is small. Therefore, even in a case, for example, where merely the amplitude is small and where the signal hardly undergoes the influence of noise, or where the amplitude becomes too small because the gain control of the AGC amplifier is not appropriately performed, the reliability of the reception signal is judged to be low. This has posed the problem that the reception signal from which correct information can be extracted only if appropriately amplified is not effectively utilized.
Besides, as stated above, the characteristic degradation occurs, not only when the amplitude is too small, but also when the amplitude is too large. In this regard, the prior-art apparatus has had the problem that only the too small amplitude is coped with, and that the too large amplitude cannot be coped with.
Further, the prior-art apparatus has had the problem that, since the weighting factor is set in accordance with the instantaneous reception amplitude, it is liable to be governed by chance, so an inappropriate weighting factor which is under the influence of noise by way of example is liable to be set.